Hypovolemic Hypoosmolar Hyponatremia

Aka: Hypovolemic Hypoosmolar Hyponatremia

advertisement

II. Pathophysiology

  1. Total Body Sodium Deficit exceeds water losses
  2. Decreased Extracellular fluid volume
    1. Increased proximal tubule fluid reabsorption
    2. Decreased distal segment flow where dilution occurs
  3. Hypovolemia stimulates non-osmotic fluid conservation
    1. Thirst
    2. ADH secretion

III. Causes: Non-Renal Losses (Sodium appropriately conserved, Urine Sodium <20 meq/L)

  1. Gastrointestinal losses
    1. Diarrhea
    2. Vomiting
      1. Also causes Metabolic Alkalosis
      2. Triggers renal bicarbonate excretion, and concomitant renal Sodium losses
  2. Third space losses
    1. Pancreatitis
    2. Pleural Effusion
  3. Skin Losses
    1. Severe burns

IV. Causes: Renal Losses (Renal inappropriate Sodium losses, Urine Sodium >20 meq/L)

  1. Diuretics (Thiazide Diuretics, Loop Diuretics)
    1. Sodium loss with overdiuresis triggers ADH release
    2. Increased ADH results in free water retention
  2. Renal Tubular Acidosis
    1. Hyperchloremic Metabolic Acidosis
    2. Increased urinary pH
    3. Fractional Excretion of Bicarbonate >15-20%
  3. Salt-losing Glomerulonephritis
    1. Chronic Renal Insufficiency on Low Sodium Diet
    2. Severe interstitial Kidney disease
      1. Polycystic Kidney Disease
      2. Medullary cystic disease
      3. Chronic Pyelonephritis
  4. Mineralocorticoid and Glucocorticoid deficiency
    1. Adrenal Insufficiency (Addison's Disease)
  5. Osmotic Diuresis (Bicarbonate, Glucose, Ketones)
    1. Excess osmotically active solutes in urine
    2. Draws increased Sodium and water renal losses
  6. Salt Wasting Nephropathy
    1. Causative agents (esp. Chemotherapy) inhibit epithelial Sodium channels resulting in Polyuria
  7. Cerebral salt wasting (head injuries, Intracranial Hemorrhage)
    1. Rare diagnosis of exclusion
    2. Associated with loss of ADH excretion and excessive Urine Output and key Urine Sodium loss
    3. Contrast with SIADH (characterized by water retention)

V. Labs

  1. Urine Sodium < 20 meq/L
    1. Non-Renal Sodium Loss (e.g. Vomiting, Diarrhea, severe burns)
    2. Other lab findings
      1. Urine Osmolality >400 mOsm/kg
      2. Fractional Excretion of Urea <35%
        1. Use instead of Urine Sodium in patients on Diuretics
        2. Carvounis (2002) Kidney Int 62(6): 2223-9 [PubMed]
  2. Urine Sodium > 20 meq/L
    1. Renal Sodium Loss (e.g. Diuretics, RTA, Adrenal Insufficiency)
    2. Other lab findings
      1. Urine Osmolality <400 mOsm/kg

VI. Differential Diagnosis

  1. Often difficult to distinguish Iso- from Hypovolemic
  2. See Isovolemic Hypoosmolar Hyponatremia

VII. Management

  1. See Hyponatremia Management
  2. Stop all Diuretics
  3. Correct non-renal fluid losses
  4. Replace Sodium deficit
    1. Calculate Total Body Sodium Deficit
    2. Use Normal Saline (0.9% = 150 meq/L)
    3. Replace one third Sodium deficit over first 6-8 hours
    4. Replace remaining Sodium deficit in next 24-48 hours

VIII. References

  1. Edwards, Yang and Mehta (2025) Crit Dec Emerg Med 39(9): 25-33
  2. Kone in Tisher (1993) Nephrology, p. 87-100
  3. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  4. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  5. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
  6. Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]

Images: Related links to external sites (from Bing)

These images are a random sampling from a Bing search on the term "Hypovolemic Hypoosmolar Hyponatremia." Click on the image (or right click) to open the source website in a new browser window. Search Bing for all related images

Related Topics in Sodium Disorders

advertisement